Circuit Labs - Electronic Circuits Collection

Author: Jean-Paul Brodier

 All microprocessors from the 8051 family have inputs and outputs that are ‘quasi-bidirectional’. This means that when power is first applied, the ports behave as inputs with a logic high level and a weak pull-up.

Glitch
When driving a relay or some other load such as an optocoupler or LED, there is a problem at power on: the NPN transistor in the common emitter connection (Figure 1) causes an undesirable excitation of the load
from the moment power is applied until the microprocessor has had the chance to turn the output low. In addition, logic high outputs are seldom able to
deliver enough current to drive the transistor into saturation because they have been designed to be active low. To solve both of these problems in one hit, we have to make the active level logic low. This can be done in three different ways: use an emitter follower as a buffer stage (Figure 2a), an inverter in a common emitter circuit (Figure 2b) or an inverter/open collector circuit (Figure 2c). The disadvantage of solution 2a is the fact that the voltage to the load is reduced. In the case of a relay with a 5-V coil there is the risk that the resulting voltage is too low. The disadvantage of examples 2b and 2c is that they require more parts.

Collector follower
That leaves the open collector buffer in the form of an IC type 7404. This solution, however, also has a few disadvantages. You do not always need all of the 6 buffers in one IC. Also, the SMD version can only handle 12 V. This is too low and dangerous if we happen to supply the load from an unregulated voltage. The solution presented here combines in one transistor the advantages of the emitter follower (inactive when power is first applied) and open collector (higher power supply voltage, lower current). This circuit has been known since the valve era by the name cascode (drive via the cathode). The goal was to reduce the Miller-effect of the internal (parasitic) capacitances. Not having the option of reducing the capacitance between the internal electrodes, a lower voltage was used instead. The cascode circuit is often used in powerful transmitters (tens of kW) to minimise the Miller-effect. This circuit was also used to limit transistor conduction and to keep the dissipation within bounds, which increased the life of bipolar transistors. This was in the IGBT and VMOS era.

The transistor conducts only when the output from the microprocessor is low (refer Figure 3). The base current is limited by resistor R. This current is determined by the current flowing through the load. When the power is switched on, both the base and emitter see the same potential, VCC, so the transistor remains blocked. One thing we have to keep in mind: we may not exceed the current rating of the microprocessor output because it has to cope with all the current flowing in the emitter of the transistor. In the case of the quite common 80C51, this maximum current is typically 3.2 mA (two LS TTL loads).

This is sufficient to drive an LED without overloading the 5-V regulator, or for driving a PNP power stage at the high side (Figure 3b). The parallel Philips PCF88574 I2C interfaces can handle 25 mA. For the Atmel AT89Cx051 as well as for the Philips P89LPC9xx the limit is 20 mA. For the latter type the cascode circuit or ‘collector follower’ is even more interesting when the outputs are configured as open-drain because the nominal voltage is only 3.6 V. In all cases we have to make sure that the maximum dissipation of the package is not exceeded. Should this be the case, then the number of open collectors required will probably justify resorting to a 7404.

A current of around 20 mA at 24 V is sufficient to energise a half Watt relay coil, which in turn can drive a load of 16 A at 230 V. For loads driven from the positive side, the voltage and current limits are determined by the power PNP (or VMOS) transistor. The cascode transistor can be a ‘digital’ type with integrated base and emitter resistors.

(Elektor Electronics - 10/2005)

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